1. Field of the Invention
The invention relates to a motor controller and an electric power steering system.
2. Description of the Related Art
In a motor controller used in an electric power steering system (EPS) etc., a drive circuit (PWM inverter) for supplying electric power for drive to the a motor based on motor control signals is formed by connecting in parallel the switching arms corresponding to the respective phases, each switching arm including a pair of switching devices connected in series. As an example of such a motor controller, one, in which an electric current sensor is provided on the lower potential side (ground side) of each of the switching arms constituting the drive circuit, is available. Refer to Japanese Patent Application Publication No. 2009-1055, for example.
In the case of the application that requires smooth rotation of the motor and excellent quietness, such as the EPS, in general, the electric power for drive is supplied to a motor with sinusoidal-waveform energization. However, such a mode of electricity supply requires feedback of the phase current values. Thus, the drive circuit, which functions as the output section that outputs the electric power for drive, is provided with electric current sensors that detect the electric current values of the respective phases.
In such a motor controller, detection of the phase current values by the electric current sensors that are provided on the lower potential side (ground side) of the drive circuit is performed at the timing at which all the switching devices on the lower potential side (lower side), which are constituent elements of the drive circuit, are turned on.
As shown in FIG. 13, the generation of the motor control signals is performed based on the result of comparison between the triangular waves (δ1, δ2) and the DUTY command values (Du, Dv, and Dw) of the respective phases that are calculated when the feedback control of the electric current is performed. In this example, two triangular waves (δ1, δ2) (δ1>δ2) that are shifted in the vertical direction are used to set the dead time for preventing the short circuit (arm short circuit) between the switching device on the higher potential side (upper side) and the switching device on the lower potential side (lower side) of the switching arm when the switching devices constituting the drive circuit are turned on and off.
When the DUTY command value(s) Du, Dv, and/or Dw is/are higher than the value of the triangular wave δ1, the motor control signals that turn on the switching device(s) on the higher potential side (upper side) corresponding to such a phase(s) are generated, and when the DUTY command value(s) Du, Dv, and/or Dw is/are lower than the value of the triangular wave δ2, the motor control signals that turn on the switching device(s) on the lower potential side (lower side) corresponding to such a phase(s) are generated. The phase current values are detected around the timing at which triangular waves δ1 and δ2 used to generate the motor control signals reach the “peak.”
However, although the electric current is detected at the timing at which all the switching devices on the lower potential side are turned on, detection of the electric current requires a certain period of time. Thus, when the DUTY command values Du, Dv, and Dw increase, a situation occurs where the on time t0, during which the switching device on the lower potential side corresponding to the relevant phase is in an on state, becomes shorter than the detection time ts required to detect the phase current value, and therefore, the electric current cannot be detected. Thus, conventionally, the upper limit value Dmax is set for the DUTY command values Du, Dv, and Dw of the respective phases in consideration of the detection time ts in order to secure the detection time ts for detecting the phase current value (the time obtained by adding the dead time td, during which both of the switching devices are turned off to prevent the arm short circuit, to the detection time ts as a margin).
However, when the upper limit value Dmax is set for the DUTY command values Du, Dv, and Dw in this way, the voltage utilization factor is reduced. For example, when the detection time ts for detecting the phase current value is approximately 4 μs, which is approximately 8% in terms of duty factor, and the dead time td is approximately 1 μs, which is approximately 2% in terms of duty factor, the upper limit value Dmax is approximately 90% (100%-8%-2%=90%). This means that approximately 90% only of the voltage that the drive circuit can output can be used.
As the method that addresses the problem of the reduction in the voltage utilization factor, the blind estimation method is available. The blind estimation method is a method, in which with the use of the fact that the total of the phase current values of the respective phases is zero, the phase current value of the electric current undetectable phase is estimated based on the phase current values of the two phases other than the electric current undetectable phase that occurs because the on time t0 of the switching device on the lower potential side becomes shorter than the detection time ts for detecting the electric current value as described above.
In the case where the electric current undetectable phase occurs in this way, however, even when the phase current values of the two phases other than the electric current undetectable phase are detected, the switching devices are turned on and off in the electric current undetectable phase, which causes the noise therefrom to be included in the phase current values of the remaining two phases, of which the electric currents are detectable, which has been the cause of the deterioration in the accuracy in detecting the electric current in the blind estimation.